The present invention relates to an optical fiber that is suitably used for wavelength division multiplexing (WDM) optical transmission system.
An optical fiber with a primary silica content and a germanium-doped core is in widespread use. For increasing the transmission capacity of optical fiber in action, a technique of performing WDM optical transmission is actively studied. In recent years, an optical transmission system using Raman amplification is also studied.
As to the Raman amplification, it is known that the maximum value of a Raman gain is obtained some 100 nm on the longer wavelength side with reference to a pumping light source wavelength. By utilizing this phenomenon, an attempt has been made to amplify WDM signal light with a plurality of pumping light sources having different wavelengths (hereinafter referred to as the xe2x80x9cwavelength multiplexed pumping light sourcexe2x80x9d).
Here, in order to prevent the pumping light on the longest wavelength side of the wavelength multiplexed pumping light source overlapping signal light of a WDM signal on the shortest wavelength side, it is required that the wavelength band of the wavelength multiplexed pumping light source is equal to or less than 100 nm.
For example, when signal light in a wavelength band of 1530 nm to 1565 nm (generally called the xe2x80x9cC-Bandxe2x80x9d) and signal light in a wavelength band of 1565 nm to 1625 nm (generally called the xe2x80x9cL-Bandxe2x80x9d) are simultaneously amplified, the maximum wavelength band of the wavelength multiplexed pumping light source becomes around 95 nm in a range of around 1430 nm to 1525 nm. Consequently, it is possible to perform Raman amplification simultaneously at C-Band and L-Band.
Also, in order to realize an optical transmission system using the Raman amplification, it is required that no unrecoverable waveform distortion occurs at a repeater and a receiving station (terminal). To this end, it is required to suppress a nonlinear phenomenon in an optical transmission line and to reduce cumulative dispersion in the optical transmission line.
Meanwhile, as to an optical transmission line used in an optical transmission system using the Raman amplification, active studies are being made on the use of a conventional single-mode optical fiber (SMF) having zero dispersion at a wavelength of around 1.3 xcexcm, an optical fiber with high refractive index of its core (such as an optical fiber (NZDSF) having small dispersion at a wavelength of around 1.55 xcexcm), and the like.
However, the effective area (Aeff) of the SMF is around 80 xcexcm2, relatively large; and its Raman amplification efficiency is low. Therefore, 1W or higher total optical power of the wavelength multiplexed pumping light source is required to provide Raman amplification. Consequently, there is a problem that this optical fiber is uneconomical.
Also, the NZDSF intrinsically has large Rayleigh scattering in comparison with the SMF described above because the refractive index of its core is high and the relative refractive index difference between the core and a cladding region is set to around 1% or the like. If an optical fiber like this having a high Rayleigh scattering coefficient is used for the Raman amplification, so-called double Rayleigh scattering tends to occur: a noise component scattered rearward is next scattered frontward and overlaps a signal. Also, the signal-to-noise characteristic (SNR) tends to be deteriorated.
Also, in order to avoid a four wave mixing (FWM) problem, the NZDSF is designed to disallow the zero-dispersion wavelength exist in a signal band or a pumping light band. However, the occurrence of another nonlinear phenomenon due to the input of high-power light becomes a problem. In many cases, a dense WDM (DWDM) transmission system is applied to a long-distance trunk system and an extremely high transmission speed is generally used in such cases. Consequently, it is required to suppress a bit error rate (BER) to an extremely low level, so that if there exists the nonlinear phenomenon problem described above, it becomes difficult to apply the NZDSF to the Raman amplification in the DWMD transmission system.
Also, as an optical fiber that is applicable to the optical transmission system using the Raman amplification, there may also be conceived a dispersion-flattened optical fiber (DFF) of which dispersion in the wavelength band of signal light takes almost a constant value.
A tangible example of the DFF is an optical fiber disclosed in Japanese Laid-Open Gazette JP 11-84159 A. This optical fiber is characterized in that the mode field diameter at a wavelength of 1550 nm is roughly equal to or more than 8.6 xcexcm (if converted into Aeff, this value becomes around 60 xcexcm2 or higher).
However, the efficiency of the optical fiber disclosed in JP 11-84159 A is still low from the viewpoint of providing the Raman amplification, although the efficiency is not so low when compared with the SMF described above. Therefore, it is impossible to significantly reduce the total power of the wavelength multiplexed pumping light source, so that the uneconomical problem still remains.
Additionally, an optical fiber of a type, differing from the NZDSF, DFF, and the like described above, is usable in a wide wavelength band, is disclosed in Japanese Laid-Open Gazette JP 2000-221352, for instance.
However, the optical fiber disclosed in JP 2000-221352 A is mainly designed to transmit signal light in both of 1.3 xcexcm and 1.55 xcexcm wavelength bands and its zero-dispersion wavelength exists in a range of 1.37 xcexcm to 1.50 xcexcm (all of the zero-dispersion wavelengths illustrated in the specific example exist on the longer wavelength side than 1.41 xcexcm). As a result, there can be occurred four wave mixing (FWM) due to the existence of the zero-dispersion wavelength in the pumping light band or in the proximity thereof like in the case of the NZDSF described above.
That is, each optical fiber described above is not suitable for providing an optical transmission system with the Raman amplification.
The present invention has an object to provide an optical fiber particularly suitable for the Raman amplification.
An optical fiber in the present invention is characterized in that an effective area at a wavelength of 1570 nm is in a range of 35 xcexcm2 to 45 xcexcm2, the absolute value of a dispersion slope at the wavelength is equal to or less than 0.04 ps/nm2/km, and a dispersion value at the wavelength is in a range of 5 ps/nm/km to 10 ps/nm/km.
The optical fiber described above has a refractive index profile containing one or more annular region between a center core and a cladding region, the maximum relative refractive index difference xcex941 (index difference between the maximum index of the center core and the index of the cladding) is made to be in a range of 0.55% to 0.7%, and the minimum relative refractive index difference xcex942 (index difference between the minimum index of an annular region and the index of the cladding) is made in a range of xe2x88x920.7% to xe2x88x920.5%.
It is desirable that the optical fiber has a refractive index profile containing one or more annular region between the center core and the cladding region and, when the outside diameter of the center core is referred to as xe2x80x9caxe2x80x9d and the outside diameter of the annular core is referred to as xe2x80x9cbxe2x80x9d, the value of b/a is in a range of 1.2 to 1.5.
Further, it is desirable that the value of xe2x80x9cbxe2x80x9d is in a range of 9 xcexcm to 12 xcexcm.
According to another aspect of the invention, the optical fiber comprises a center core, cladding and annular regions between the center core and cladding and has characteristics of xcex941xe2x89xa60.7% where xcex941 is the relative index difference between the maximum index of center core and the index of cladding at an optical signal wavelength of 1570 nm and Aeffxe2x89xa645 xcexcm2 where Aeff is the effective area. The optical fiber is further defined by the following characteristics;
xcex941xe2x89xa70.55%
xe2x88x920.7%xe2x89xa6xcex942xe2x89xa6xe2x88x920.5%
1.2xe2x89xa6b/axe2x89xa61.5
9 xcexcmxe2x89xa6bxe2x89xa612 xcexcm,
where xcex942 is the relative index difference between the minimum index of annular region and the index of cladding, xe2x80x9caxe2x80x9d is a diameter of the center core and xe2x80x9cbxe2x80x9d is the outside diameter of the annular region.
As additional characteristics, Aeff is larger than or equal to 35 xcexcm2, and an absolute value of dispersion slope and a dispersion at a wavelength of 1570 nm are respectively less then or equal to 0.04 ps/nm2/km and in the range of 5 ps/nm/km to 10 ps/nm/km.
The present invention configures a new optical transmission system comprising an optical signal transmitter, optical signal receiver optical transmission line, pumping light source and optical multiplexer for introducing a pumping light from the source into the optical transmission line to Raman-amplify an optical signal on the optical transmission line, wherein at least part of the optical transmission line comprises the optical fiber as defined either one of the above.
In the optical transmission system, the optical transmission line comprises a first optical transmission line of optical fiber as defined in the above (e.g., the optical fiber lengthxe2x89xa6100 km) and a second optical transmission of another type of optical fiber.